Just how unlikely is it that Luke would have accidentally landed on Dagobah conveniently close to where Yoda happened to be?

There is no canonical information on the size of Dagobah that I could uncover, so if we assume it's roughly Earth-sized we end up with a surface area of about 500 million square kilometres. Let's give Luke a big target area and say he would have bumped into Yoda if he'd landed within a target zone of one square kilometre. I'd say that's generous, given the terrain and the fact that we never see him wander more than a few metres from his X-Wing before Yoda bumps into him. That's a one-in-500-million chance.

Of course you could just say that the Force helped him. But then you wouldn't have a comic today.